@Ribath for below-knee prosthetics, some of them are held in place by a tight-fitting sleeve that goes above the knee, which is enough to secure it (partially because the joint makes it harder for the sleeve to slide, and the sleeve's made to not slide anyway). So in her case, her above-knee stockings are probably medical and just made to look like normal stockings a girl would wear.
It's prob not a 100% accurate depiction (like her stockings should be a bit less floppy), but it doesn't seem too outlandish to me.
Here's a vid where a guy shows off basically the same thing she has, except he has a liner too (which isn't necessary, but it helps+reduces chafing) https://www.youtube.com/watch?v=hwUF_DPXLPA